The broad objective of this application is to examine the population dynamics of individual antigen-specific T cells during the rejection of a cardiac allograft and the development of allograft tolerance. The application plans detailed examination of the allograft and lymphoid tissues in recipient mice using conventional morphologic examination, a series of immunohistochemical studies using monoclonal antibody staining, an extensive panel of in situ hybridization studies focused on cytokine gene expression, and correlative analysis of cytokine gene expression using QC-RT-PCR. The primary system for analysis will be adoptive transfer of CD4 positive T cells with single antigen-specificity into immunodeficent syngeneic mice, such as SCID or rag knockout animals, that are transplanted with an alloantigenic cardiac graft. The first specific aim is to determine whether the mechanisms of rejection vary dependent on cytokine expression pattern and TCR-specificity and avidity in adoptive transfers of T cells into transplanted mice. Both T cell clones and TCR-transgenic T cells using alloreactive TCRs will be used for these studies. The roles of precise TCR specificity in the cytokine gene expression pattern by particular T cells can be distinguished using adoptive transfer of TCR transgenic cells differentiated in vitro into distinct functional subsets. The second specific aim is to produce several transgenic mice which express an immunoregulatory molecule under the control of a cardiac-specific promoter to probe the role of the allograft histological microenvironment on the progress of the immune response mediated by different kinds of T cells.